Solar concentration plant for the production of superheated steam

ABSTRACT

A solar concentration plant which uses water/steam as a fluid, in any thermodynamic cycle for the exploitation of process heat, comprising an evaporation subsystem, where saturated steam is produced, a superheater subsystem through which the steam reaches the required conditions of pressure and temperature at the turbine inlet, and an attemperation system interconnected by a drum. A field of heliostats is pointed towards either of the subsystems (evaporator or superheater), in such a way to control both the pressure within the drum and the outlet temperature of the superheated steam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/156,816 filed Jun. 5, 2008 the disclosure of which is incorporated byreference herein.

The present invention relates to solar concentration plants with aphysical separation between the evaporator and the superheater and witha dynamic control capable of adapting the heliostat field, in order toproduce superheated steam in a controlled, efficient manner, in order tothus assure the continued durability and normal operation of said solarplant in its different applications: the production of electricity, theproduction of process heat, the production of solar fuels and theirapplication to thermochemical processes.

BACKGROUND OF THE INVENTION

While solar radiation is a source of thermal energy of a hightemperature at its origin, the use of the same under the conditions ofthe flow which reaches the surface of the earth destroys practically allits potential for conversion into work due to the drastic reduction inthe temperature available in the flow. For this reason, thermoelectricsolar plants and optical concentration systems are used, which achieve agreater density of the flow and thus higher temperatures.

Currently, there exist three main different technologies developed foruse in Solar Facilities, these being: the central receiver type,cylindrical-parabolic collectors and Stirling discs. All of these makeuse only of the direct component of solar radiation, which makes itnecessary for them to feature solar tracking devices:

1. Central receiver-type systems (3D) use mirrors of a large surfacearea (40-125 m2 per unit) called heliostats, which feature a controlsystem in order to reflect the direct solar radiation onto a centralreceiver located at the top of a tower. By means of this technology, theconcentrated solar radiation heats a fluid in the interior of thereceiver to a temperature of as much as 1000.degree. C.; this thermalenergy may subsequently be used for the generation of electricity.

2. Regarding cylindrical-parabolic collectors (2D), the direct solarradiation is reflected by cylindrical-parabolic mirrors whichconcentrate the same onto a receiving or absorbing tube through whichthere circulates a fluid which is heated as a consequence of theconcentrated solar radiation which falls on the same at a maximumtemperature of 400.degree. C. in this way, the solar radiation isconverted into thermal energy which is used subsequently for thegeneration of electricity by means of a Rankine water/steam cycle.

A variation in this technology is embodied in linear Fresnelconcentration systems, in which the parabolic mirror is replaced by aFresnel discretization with mirrors of smaller dimensions, which may beflat or may feature a slight curvature at their axis, and by means ofcontrolling their axial orientation allow the concentration of solarenergy on the absorbing tube, which in this type of applications usuallyremains static.

3. Parabolic Stirling disc systems (3D) use a surface area of mirrorsinstalled on a parabola of revolution which reflect and concentrate therays of the sun onto a focal point, at which the receiver is located; insaid receiver the working fluid of a Stirling engine is heated; thisengine in turn operates a small electrical generator.

In central-receiver systems, water-steam technology is currently themost conventional. The steam is produced and superheated in the solarreceiver at temperatures of approximately 500.degree. C. and 10 MPa (100bar) and is sent directly to the turbine. In order to reduce the impactof transitional conditions (the passing of clouds, etc.) a storagesystem is used (melted salts or a rock/oil thermocline). This conceptwas the first to be tested due to its permitting the transposition ofthe habitual techniques of thermal power plants and to its permittingthe direct access of the steam issuing from the solar receiver into theturbine.

The use of superheated steam may allow the implementation ofthermodynamic cycles of a higher efficiency in power plants.

The difficulty of solar technology for the production of superheatedsteam lies in the demanding conditions of temperature at which thereceiver must operate. The walls of its pipes are continuously subjectedto thermal cycles between ambient temperature, the temperature of thesteam with which the receiver is fed (250 to 310.degree. C.), and thetemperature necessary at its wall for the generation of superheatedsteam at 540.degree. C. or nearly 600.degree. C. Unlike the receiverswhich generate saturated steam, which operate at a temperature which isalmost uniform throughout their sections (330.degree. C.), superheatedsteam receivers increase the temperature of their pipes in accordancewith their greater proximity to the steam outlet zone.

The difficulties encountered in the experiments carried out onsuperheated steam receivers in the eighties were centered on two mainaspects:

A lack of controllability of the system, especially when faced withtransitional conditions, the passing of clouds, etc., due mainly to thebad thermal properties of the superheated steam. In both receivers, themost frequent structural fault was the appearance of cracks. The thermaltension due to the great differences in temperature brought about theappearance of cracks in the interstitial weld between subpanels. Thissituation occurred fundamentally during downtime, when the water in asubpanel, at saturation temperature, flowed upwards, where thetemperature was still that of the superheated steam, while in theadjacent subpanel this phenomenon did not occur

The problem of operating at high pressures, which made necessary greaterthicknesses of the walls of the piping; this, when the time came totransfer high densities of power to the heat-carrying fluid, necessarilyimplied high thermal gradients.

The invention proposed below therefore deals with the agglutination ofthe advantages of the use of high-temperature steam, resolving theexisting risks, achieving a greater control over the plant and thusfavouring the stability and durability of the same.

DESCRIPTION OF THE INVENTION

The present invention relates to a plant and a procedure which useswater/steam as a heat-carrying fluid for the production of superheatedsteam in any thermodynamic cycle or process heat exploitation system,comprised of an evaporation subsystem where saturated steam is producedunder the conditions of pressure of the system, and a superheatersubsystem where the steam reaches the required conditions of pressureand temperature for entry into the turbine. With regard to otherpreceding proposals which located the superheater subsystem modules inthe very close proximity of (when not superimposed on) the evaporatorsubsystem modules, the strategic development proposed herein is based onthe physical and independent separation of the evaporator and thesuperheater.

The fact of separating the evaporation stage from that of superheatingreduces the technological risk as, due to there not being a change ofphase in the receiver itself, neither are there any problems of highthermal gradients derived from the different film coefficients of bothphases. In addition to physically and independently separating theevaporator and the superheater by means of the inclusion of anintermediate drum, this incorporates the carrying out of a strategiccontrol of the pointing of the heliostat field which is independent forboth receiving subsystems, the evaporator subsystem and the superheatersubsystem. This strategic control consists of a dynamic control, capableof adapting the heliostat field, in order that after the provision ofenergy, optimal conditions of heat and pressure for entry into theturbine are maintained stable. To do this, the heliostat field ispointed towards one receiver or the other (evaporator or superheater)depending on the current need. By this it should be understood thatthere is a possibility of carrying out the pointing of individualheliostats or groups of the same, towards either the evaporator receiveror the superheater receiver, in such a way that they control jointlyboth the pressure in the drum and the superheater outlet temperature. Inthis way, part of the heliostat field will be directed towards theevaporator and the other part towards the superheater, thus achievinggreater control over the plant and greater stability of the same.

In central receiver technology, the receiver is located at the top ofthe tower, and the heliostats concentrate the solar energy onto thesame. The energy exchange occurs in the receiver, transferring thephotonic energy of the concentrated beam of light coming from the fieldof heliostats to a heat-carrying fluid, increasing its enthalpy. Thereare many different ways of classifying the receivers. If we classify thereceivers in accordance with their geometry, we may define the “cavity”type receivers as those which are located at the top of the tower withina hollow or cavity; in this way, the thermal losses due to radiation orconvection are minimised. The receivers may be constituted in differentways, the most common for the direct generation of steam in centralreceiver systems being tubular solar panels.

This receiver is designed in accordance with a particular geometricconfiguration, generally defined by a series of subpanels constituted bythe array of tubes which form the evaporator or the superheater.

The solar plants disclosed in the present invention are comprised of athree-dimensional solar concentration system with a central tower whichincludes:

a) A receiving evaporation subsystem with an evaporator for theevaporation of water

b) A receiving superheater subsystem, with a number of superheaters(primary and secondary or final) for the superheating of the steamproduced, located in the same or different cavities and physicallylocated independently from the evaporators

c) A drum as a means of connection between the two subsystems, that ofevaporation and that of superheating

d) A strategic control for the pointing of the heliostat field towardsthe evaporator receivers and the superheater receivers

These may include between the primary superheater and the secondary orfinal superheater a series of attemperators or an attemperation system,thus achieving a more precise control over the pressure- andtemperature-related conditions of the superheated steam at the outlet ofthe superheater receiver.

To complement the description carried out above and with the aim ofaiding a better understanding of the characteristics of the invention, adetailed description of a preferred embodiment will be carried outbelow, by means of a set of drawings which accompany this specificationand in which, by way of guidance and of a non-limitative nature, thefollowing has been portrayed:

FIG. 1 portrays a diagram of a twin-cavity tower, the first cavityhousing an evaporator and the second cavity housing a superheater.

FIG. 2 portrays a diagram of a twin-cavity tower, the first cavityhousing an evaporator and the second cavity housing two superheaters.

FIG. 3 portrays a diagram of a single-cavity tower, with a single cavityhousing an evaporator and a superheater.

FIG. 4 portrays a diagram of a single-cavity tower, with a single cavityhousing an evaporator and two or more superheaters.

FIG. 5 portrays a diagram of the attemperation system to control thetemperature of the steam at the outlet of the final superheater.

In this figure, the numeric references correspond to the following partsand elements:

-   -   1.—Heliostats.    -   2.—Central tower.    -   3.—Cavity.    -   4.—Evaporator.    -   5.—Drum.    -   6.—Primary superheater.    -   7.—Secondary superheater.    -   8.—Fossil backup system    -   9.—Saturated steam from drum.    -   10.—Attemperation system.    -   11.—Superheated steam.

In the application of the concept of the plant of this invention,central tower and receiver technology is used to carry out a solarsuperheating process on damp or saturated steam.

As may be seen in FIG. 1, this solar plant is comprised of athree-dimensional solar concentration system with a central tower (2)which features two cavities (3), one of these housing an evaporatingreceiver (4) for the evaporation of water, and the other featuring asuperheating receiver (6) for the superheating of the steam produced,and a field of heliostats (1).

In order to comply with the aim of superheating, it is proposed to carryout a series of heliostat pointing strategies by means of a dynamiccontrol system adapted to the field of heliostats in such a way that theconditions of temperature and pressure of the steam at the inlet of theturbine may be maintained constant, directing part of the field ofheliostats towards the evaporator (4) and the other part towards thesuperheater (6). That is to say, the proposal is the use of concentratedradiation from a percentage of the field of heliostats for theevaporation phase, and the use of the remainder of the field ofheliostats for the concentration of radiation destined for the steamsuperheater (6) until temperatures even higher than 550.degree. C. arereached, in such a way that the two subsystems (evaporator andsuperheater) are separate within the receiver. For the preheating of thewater which is to be evaporated a fossil fuel backup system (8) isincorporated.

In FIG. 2, a detail of a receiver with two cavities may be seen; herethe superheating is carried out in two stages, by means of a primarysuperheater (6) and another secondary superheater (7), both of theselocated in a second cavity (3). The steam coming from the evaporator(4), located in a first cavity (3) and in which the water reaches itssaturation temperature, changing to its steam stage, is superheated inthe superheater until temperatures in the region of 550.degree. C arereached. Located between the two elements (evaporator (4) andsuperheaters (6) and (7)) there will be a drum (5) whose purpose it isto separate the water in a liquid state from the steam which will enterthe superheater.

FIGS. 3 and 4 portray a tower (2) with a single cavity in which the twosubsystems, evaporator and superheater, are located. FIG. 3 portrays thesimplest case in which there is a single superheater (6) and anevaporator (4). In the case of FIG. 4, two superheaters are included,primary (6) and secondary (7), and an evaporator (4).

In the cases where the tower contains two or more superheaters (FIGS. 2and 4), the saturated steam (9) coming from the drum (5), after passingthrough the primary superheater (6), is subjected to a pressure andtemperature control process by means of an attemperation system (10), asmay be seen in FIG. 4. Subsequently, it passes through the secondary orfinal superheater (7), thus obtaining superheated steam (11) under morecontrolled conditions of pressure and temperature.

The aim of the installation disclosed above is to achieve a moreefficient, less costly result than the current solar concentrationtechnologies, clearly improving the controllability of the plant whenfaced by transitional conditions, and the durability and stability ofthe same. The final control of the plant envisages both the combined useof all these control strategies and the independent use of the same, inaccordance with the operating mode in question.

Its application is particularly suited in the fields of the productionof electricity, of process heat and solar fuels, as well as in thermalprocesses.

1. A facility for the production of superheated steam whereinwater/steam is superheated in a three-dimensional solar concentrationsystem in a central tower, the facility comprising: a) an evaporationreceiver subsystem; b) a superheater receiver subsystem for thesuperheating of the steam produced that is physically locatedindependently from the evaporation subsystem; c) a drum flow connectingbetween the evaporation receiver subsystem and the superheater receiversubsystem; and d) a strategic controller for the pointing of a pluralityof heliostats in a field of heliostats selectively between theevaporation receiver subsystem and the superheater receiver subsystem insuch a way that the subsystems jointly control both the pressure withinthe drum and an outlet temperature of superheated steam producedtherein.
 2. The facility for the production of superheated steam, ascalled for in claim 1, wherein the superheater receiver subsystem has atleast first and second superheaters.
 3. The facility for the productionof superheated steam, as called for in claim 2, including anattemperation system between the first and second primary superheaters.4. The facility for the production of superheated steam, as called forin claim 1, wherein the superheater receiver subsystem has onesuperheater.
 5. The facility for the production of superheated steam, ascalled for in claim 1, wherein the evaporation receiver subsystem hasone evaporator.
 6. A procedure for the production of superheated steam,characterised in that it consists of superheating water/steam by meansof a three-dimensional solar concentration system in a central tower (2)which includes: a) an evaporation receiver subsystem; b) a superheaterreceived subsystem for the superheating of the steam produced,physically located independently from the evaporation subsystem; c) adrum (5) as a means of connection between the two subsystems, that ofevaporation and that of superheating; and d) a strategic control for thepointing of the field of heliostats (1) towards either subsystem(evaporator or superheater), with individual or group pointing of theheliostats, in such a way that they jointly control both the pressurewithin the drum (5) and the outlet temperature of the superheated steam(11).
 7. A procedure for the production of superheated steam, as claimedin claim 6, characterised in that the superheater subsystem features twoor more superheaters (6, 7).
 8. A procedure for the production ofsuperheated steam, as claimed in claim 7, characterised in that itfeatures an attemperation system (10) between the primary superheater(6) and the secondary or final superheater (7).
 9. A procedure for theproduction of superheated steam, as claimed in claim 6, characterised inthat the superheater subsystem features one superheater (6).
 10. Aprocedure for the production of superheated steam, as claimed in claim6, characterised in that the evaporation subsystem features oneevaporator (4).